AN9768 Datasheet, PDF (4/8 Pages) Littelfuse – Transient Suppression Devices and Principles

Application Note 9768
Transient Suppressors Compared
Because of diversity of characteristics and nonstandardized
manufacturer specifications, transient suppressors are not
easy to compare. A graph (Figure 3) shows the relative volt-
ampere characteristics of the four common devices that are
used in 120V AC circuits. A curve for a simple ohmic resistor
is included for comparison. It can be seen that as the alpha
factor increases, the curve's voltage-current slope becomes
less steep and approaches an almost constant voltage. High
alphas are desirable for clamping applications that require
operation over a wide range of currents.
It also is necessary to know the device energy-absorption
and peak-current capabilities when comparisons are made.
Table 2 includes other important parameters of commonly
used suppressors.
Standby Power - The power consumed by the suppressor
unit at normal line voltage is an important selection criterion.
Peak standby current is one factor that determines the
standby power of a suppressor. The standby power
dissipation depends also on the alpha characteristic of the
device.
As an example, a selenium suppressor in Table 2 can have a
12mA peak standby current and an alpha of 8 (Figure 3).
Therefore, it has a standby power dissipation of about 0.5W
on a 120V_RMS line (170V peak). A zener-diode suppressor
has standby power dissipation of less than a milliwatt. And a
silicon-carbide varistor, in a 0.75” diameter disc, has standby
power in the 200mW range. High standby power in the lower
alpha devices is necessary to achieve a reasonable
clamping voltage at higher currents.
1000
800
500
400
300
200
100
1
SILICON CARBIDE VARISTOR
(α ≈ 5)
SELENIUM 2.54cm
(1") SQ (α ≅ 8)
SILICON POWER
TRANSIENT SUPPRESSOR
(ZENER) (α ≅ 35)
LITTELFUSE VARISTOR
(20mm DIA.)
(α > 25)
2 3 4 5 8 10
20 30 40 50 80 100
INSTANTANEOUS CURRENT (A)
FIGURE 3. V-I CHARACTERISTIC OF FOUR TRANSIENT
SUPPRESSOR DEVICE
The amount of standby power that a circuit can tolerate may
be the deciding factor in the choice of a suppressor. Though
high-alpha devices have low standby power at the nominal
design voltage, a small line-voltage rise would cause a
dramatic increase in the standby power. Figure 4 shows that
for a zener-diode suppressor, a 10% increase above rated
voltage increases the standby power dissipation above its
rating by a factor of 30. But for a low-alpha device, such as
silicon carbide, the standby power increases by only 1.5
times.
50
20
10
5
2
1
0.5
0.2
0.1
96
α = 35
ZENER ^{DILOITDTEEOLRFUCSLUESVTAERRISTOR}
1" ^SELENIUM
SILICON CARBIDE
α = 25 α = 8 _{α = 4}
98
100
102 104 106
108 110
PERCENT OF RATED VOLTAGE
FIGURE 4. CHANGES IN STANDBY POWER ARE
CONSIDERABLY GREATER WHEN THE
SUPPRESSOR'S ALPHA IS HIGH
Typical volt-time curves of a gas discharge device are shown
in Figure 5 indicating an initial high clamping voltage. The
gas-discharge suppressor turns on when the transient pulse
exceeds the impulse sparkover voltage. Two representative
surge rates 1kV/µs and 20kV/µs are shown in Figure 5.
When a surge voltage is applied, the device turns on at
some point within the indicated limits. At 20kV/µs, the
discharge unit will sparkover between 600V and 2500V. At
1kV/µs, it will sparkover between 390V and 1500V.
6000
5000
4000
3000
2000
1000
800
600
500
400
300
300
200
10^-9
MAXIMUM _VOLTAGE
MINIMUM _VOLTAGE
9.5mm OD, 230V
GAS-DISCHARGE SUPPRESSOR
10^-8
10^-7
10^-6
10^-5
10^-4
SHORT-TIME SURGE RESPONSE (S)
10^-3
FIGURE 5. IMPULSE BREAKOVER OF A GAS-DISCHARGE
DEVICE DEPENDS UPON THE RATE OF
VOLTAGE RISE AS WELL AS THE ABSOLUTE
VOLTAGE LEVEL
10-105